1. Field of the Invention
The present invention relates to a method for controlling pitch troubles in a process for the production of mechanical pulp and/or of mechanical pulp-containing paper.
2. Prior Art
Mechanical pulp, such as groundwood pulp (hereinafter referred to as GP), refiner groundwood pulp and thermomechanical pulp is produced by the simplified mechanical treatment of grinding logs or chips by means of a grinder or refiner. The mechanical pulp, which has a defect of providing lower strength, exhibits advantages of providing higher yield, lower costs and higher opacity, compared with a chemical pulp produced by various chemicals such as alkaline chemicals. On the contrary, the wood constituents remain unchanged in the mechanical pulp. Mechanical pulp used in the present invention differs from SCP which is always produced by the digestion process for delignification. However, mechanical pulp does not include SCP. Further, the mechanical pulping process does not include a digestion process.
In general, a wood contains circa 1-10% of pitch, organic solvent-soluble extractive, besides three major constituents composed of cellulose, hemicellulose and lignin. The amount and quality of pitch vary with wood species. However, it is well-known that softwood, i.e. a main raw material of mechanical pulp, contains a large amount of pitch constituents such as fatty acids, resin acids, glycerides, etc. In the pulping process, these pitch-constituents are released from pulp, wherein they exist in state of liberation, deposition on fiber surface, and covering thereon. That is, they transform from a fixed state to a liberated state. Pitches suspended in reused water (white-water) are microparticles of about 0.2-2.mu., which are known as the so-called "colloidal pitch".
In general, a paper is manufactured as follows: Pulps are prepared in various pulping processes, are treated in a screening process, are mixed with different auxiliaries to prepare a paper furnish. A paper is made from the paper furnish on a paper machine. In a series of processes, the liberated pitch or the deposited pitch is accumulated in pipes, tanks, wire part, or press part, thereby causing the so-called pitch troubles, such as paper contaminations or paper brokes. In the production of a paper containing a large amount of mechanical pulp, pitch troubles occur frequently. As one of the conventional methods for controlling pitch troubles, there is the so-called seasoning, in which logs after felling are held outside and seasoned in a long period. As another method therefor, there is an addition of surface-active agent in a pulping or paper-making process, as demonstrated in Japanese Patent Publication No.50-22606, wherein the surface active agent includes, for example, alkylether of polyoxyethylene with various addition moles, alkyl phenyl derivatives, chemicals containing them as a main ingredient.
Although the above seasoning method causes the change and the decrease of pitch constituents in wood mainly by air-oxidation, it requires a broad area and a long period, for example, 3-6 months, or more. Accordingly, it is practically difficult to avoid the pitch troubles only by the above seasoning method.
On the other hand, the addition of surface active agent, which is said to disperse the pitch particles and to prevent the pitch deposits, is not recognized as a substantial method for controlling pitch troubles.